The present invention relates to a fiber-reinforced metallic composite material (hereinafter, referred to as "FRM"). More particularly, it relates to FRM which comprises a zinc/aluminum or zinc/magnesium alloy reinforced with an inorganic fiber containing two or more components selected from carbon (as a simple substance), metal oxides, metal carbides, metal nitrides and metal borides.
Recently, because of rapid technical development in many industrial fields such as aerospace, atomic power, automobiles and the like, there has occurred a strong demand for new materials of long life that are lighter in weight superior in mechanical properties such as strength, modulus of elasticity and the like as compared with the conventional materials mainly made of steel, and that are employable in high-temperature or low-temperature regions.
As one of the materials meeting such demand, there is proposed FRM which is produced by reinforcement of metals with inorganic fibers or whiskers of relatively small specific gravity, and active studies are made on the FRM. As the inorganic fibers or whiskers which are used as reinforcing materials for FRM, boron fibers, carbon or graphite fibers, alumina fibers, silicon carbide fibers, alumina whiskers and the like have so far been used.
These reinforcing fibers for FRM, however, have some drawbacks. For example, boron fibers are of high strength, but are poor in flexibility because of the large diameter, such as about 100 .mu.m, and, therefore, are inferior in fabricability. Boron fibers are easy to react with practical metals such as aluminum, magnesium and the like, easily forming boron compounds at the fiber/matrix interface at a relatively high temperature, which disadvantageously results in a reduction in FRM strength. Accordingly, the fiber surface is usually coated with silicon carbide or the like in order to inhibit the progress of this reaction. This method succeeds to some extent, but still has many drawbacks.
Carbon or graphite fibers are also of high strength and, high elasticity. However, they are easily oxidizable in air, and hence, when aluminum alloy is used as the matrix metal, brittle layers of Al.sub.4 C.sub.3 are formed at the fiber/matrix interface, resulting in the strength reduction of the composite materials. Furthermore, carbon fibers cause an electrocorrosive reaction at the fiber/matrix interface due to its good electrical conductivity, which results in a reduction in fiber strength. Carbon fibers, therefore, have a drawback in that they are easily corroded, for example by saline water.
Besides, carbon and liquid-phase aluminum are poor in wettability with each other. Consequently, for improving the wettability with matrix metals as well as inhibiting the foregoing reaction at the fiber/matrix interface, coating of carbon fiber surfaces with metals or ceramics is now actively studied with some degree of success. Carbon fibers, however, generally have a small diameter, such as less than 10 .mu.m, and, therefore, it requires a higher level of coating technique and high cost to form uniform and even coatings on all the surfaces of a large number of the fibers. It can be said, therefore, that carbon fibers, in spite of their excellent properties, still have great problems to be solved for use as metal-reinforcing fibers.
Alumina or boron carbide whiskers are very high in both tensile strength and modulus of elasticity. But, mass production of whiskers of uniform diameter and length is difficult, which is the main reason for its high cost. When the alumina whisker is processed into composite materials together with metals, the foregoing drawback, i.e. reaction with matrix metals, is not observed since it has the structure of .alpha.-Al.sub.2 O.sub.3. But on the other hand, because of its poor wettability with matrix which facilitates the formation of porosity in the composite materials, the alumina whisker has a drawback of lowering the physical properties of the composite materials.
Metallic fibers such as stainless steel fibers, particularly those having an average diameter of 8 to 15 .mu.m, are rich in flexibility. However, they have a specific gravity of about 8.0 g/cm.sup.3 which is not useful in lightening the weight of FRM. Besides, when a molten alumina is used as matrix, it easily reacts with the fibers to cause a strength reduction of the composite materials.
Suitable kinds of matrix metals vary with the utility of FRM. For example, when weight-lightening is especially required, magnesium, aluminum or their alloys are mainly used, and when thermal resistance is especially required, copper, nickel, titanium or their alloys are mainly used. Among these matals, FRM that contains aluminum, magnesium or their alloys as a matrix metal is well studied and made on a trial basis.
The design of the bonding strength at the fiber/matrix metal interface is also an important factor to provide practical FRM. The bonding strength at the interface must be controlled to an optimum degree. One of the methods for obtaining such a state is surface treatment of the fiber, and the other method is to add a trace amount of other elements to the matrix to control the bonding strength. The former method, however, requires a considerable higher level of technique in ensuring uniform and even coatings on all the surfaces of a large number of fibers, and also involves a high cost. It is also very difficult to simultaneously control the bonding strength at the fiber/coating layer and coating layer/matrix metal interfaces formed by the surface treatment to an optimum degree. In the latter method wherein a trace amount of element is added, distribution of the added element in the vicinity of the fiber surface varies delicately depending upon the amount or kind of the element to be added, with which change the bonding strength at the fiber/matrix interface also changes. Thus, in these two methods, the bonding strength is not necessarily easily controlled, which causes difficulty in the quality control of FRM, especially in commercial scale production.